Other Worlds
A/Prof. Orsola De Marco
9850 4241
[email protected]
Assignment 2
• Friday September 3, 4PM posted on Blackboard
• Deadline 2 weeks later in the boxes
• Returned October 4, after term break
The Moon Practical
• If you have not done it, read the unit outline where
there is a link to how to do the practical on line.
In last class: the terrestrial
planets I
• A few more things about the Moon to finish last week’s
• Orbital properties of Mercury (M), Venus (V) and Mars (M).
• Naked eye observations, early telescopes (and early
theries) and new observations.
• Spins and rotations and hot to measure them.
• Mercury mystery!
• The terrain of Venus and Mars
• Plate tectonics on Venus and Mars?
• Volcanos on Venus and Mars (and Earth!)
• The atmospheres of Venus and Mars (and Earth!)
• Seasons on Mars
• Evolutions of the atmospheres
In this class: the terrestrial
planets II
• Plate tectonics on Venus and Mars?
• Volcanos on Venus and Mars (and Earth!)
• The atmospheres of Venus and Mars (and Earth!)
• Seasons on Mars
• Evolutions of the atmospheres
• Water on Mars
• Life on Mars?
Why is Mercury’s spin period exactly
2/3 of its orbital period?
Tides again!!!
Venus before the space age
• Average Venus temperature with no atmosphere ~40 C.
• Atmosphere was known – possibly keeps the planet at a different temperature…
• Water Adams and Theodore Dunham (1932) saw CO2 in spectrum – greenhouse!!
• Cloud of Venus reflect a lot (see high albedo) maybe that keeps the greenhouse
in check.
• Answers have to wait for space probes….
Venus from space
Mariner 2 (1962) first successful mission
to another planet:
• It detects microwave radiation and finds Venus
• Hot (>400 C)
• and Dry
Venera 7 (1970; Russian) first lander, confirms
Venus as a desert searing world (this picture is
actually from Venera 13)
Mars before the
space age
Percival Lowell (American) 1855-1916
HST image
Viking Orbiter image
Cratered surface. Craters had escaped detection.
Some of the surface of Mars must be very old.
Mars changing colour due to winds,
not seasonal vegetation changes!
Topography of Venus
(radar altimeter from Magellan)
Topography of Mars (laser measurements
from the Mars Global Surveyor)
Highlands are old and cratered, lowlands are younger and not heavily crated.
Venus’ surface
• Venus surface has only 15% the number of
craters as the lunar maria, indicating an age
of only 500 Myr.
• There is plenty of evidence for active
volcanoes and tectonic activity, but that
activity has stopped.
• It looks like Venus goes through resurfacing
episodes, or more likely, that the tectonic
activity is on-going but is distributed evenly.
• Venus has as much heat as Earth and should
have tectonic activity.
• Possibly its thinner crust means that the inner
heat can break the surface more often and more
evenly. We call
this “flake”
• Venus tectonic
activity is local, with
local volcanos and
local upwelling, but
no large scale mountain
ranges and ridges.
Plate tectonics vs.
“flake” tectonics
• Mars has no earth-like plate tectonics. There
are no features similar to ridges and ranges.
• The crust is 40-70 km thick (on Earth it is
5-35 km). Too thick to allow plate tectonics.
• It did have tectonic
activity a long time
ago: the Tahrsis rise
was a large magma
rise, which also
cracked the crust and
made the Valles
Press release 28 August 2010 from Mars Express (ESA)
• Orcus Patera (defined but irregular volcanic craters)
• Elliptical depression, 380 km long; 1800 m rims
• Near Olympus mons
• Created by oblique impact?
• Shaped by plate tectonic (presence of “graben”)?
Venusian volcanos
• 10 million years old!
• Present day volcanic activity
• Sulfuric compounds in the
atmosphere at the 0.015%
level (which is high compared
to Earth.)
Volcanos on Mars (and Venus)
cannot be caused by
subduction. They must be
caused by hot spots (like
some volcanos on Earth – e.g.,
The lack of plate movement
on Mars means that the
volcano had time to grow
Most volcanos are old but
Olympus Mons is very young,
Olympus Mons 24 km high –
compare to Mona Loa (Hawaii) 8 km high
Venera 13 (Soviet; 1981) one of the first landers
Venera 13 measured a temperature high enough to melt lead and
pressures of 90 bars!
Comparison of the atmospheric structures
of Earth, Venus and Mars
Venus’ clouds have a 4 day rotation speed – the
planet has a 243 day spin period. WHY?
Let’s take a look at Mars’ atmosphere
Residual ice cap may contain water ice.
Very fine dust found by Viking 1
lander is easy to lift up and
Create dust storms.
Martian dust devils
Caused by warm air rising and carrying dust up in the air. They are very
large on Mars due to the fineness of the dust, and they can be seen from orbit.
Outgassing of CO2 (H2O, N2 and SO2) from
volcanos happened on Earth, Venus and Mars,
originating thick atmospheres
On Earth
On Venus
On Mars
All terrestrial planets
start with heavier
Earth: liquid water
modest CO2
Venus: initial H2O
Greenhouse effect.
More heat and no
tectonics, so more CO2.
Hence runaway
Mars: H2O froze and
rained to the ground.
Temperature dropped
even more. CO2
remained partly
in the atmosphere (not
enough to warm).
Key Ideas
• Motions of Mercury, Venus, and Mars in the Earth’s Sky:
Mercury and Venus can be seen in the morning or evening sky
only, while it is possible to see Mars at any time of night
depending on its position in its orbit.
• At their greatest eastern and western elongations, Mercury is
only 28° from the Sun and Venus is only 47° from the Sun.
• The best Earth-based views of Mars are obtained at favorable
oppositions, when Mars is simultaneously at opposition and
near perihelion.
Key Ideas
• Rotation of Mercury, Venus, and Mars: Poor telescopic views
of Mercury’s surface led to the mistaken impression that the
planet always keeps the same face toward the Sun (1-to-1
spin-orbit coupling).
• Radio and radar observations revealed that Mercury in fact
has 3-to-2 spin-orbit coupling: The planet rotates on its axis
three times every two orbits.
• Venus rotates slowly in a retrograde direction. Its rotation
period is longer than its orbital period.
• Mars rotates at almost the same rate as the Earth, and its
rotation axis is tilted by almost the same angle as the Earth’s
Key Ideas
• Mercury’s Surface, Interior, and Magnetic Field: The
Mercurian surface is pocked with craters, but there are
extensive smooth plains between these craters.
• Long cliffs called scarps meander across the surface of
Mercury. These probably formed as the planet’s crust cooled,
solidified, and shrank.
• Mercury has an iron core with a diameter equal to about 3⁄4
of the planet’s diameter. By contrast, the diameter of the
Earth’s core is only slightly more than 1⁄2 of Earth’s diameter.
• Mercury has a weak magnetic field, which indicates that at
least part of the iron core is liquid.
Key Ideas
• Comparing Venus and Mars: Most of the surface of Venus is
at about the same elevation, with just a few elevated regions.
On Mars, the southern highlands rise several kilometers
above the northern lowlands.
• Venus has a thick atmosphere and a volcanically active
surface. Mars has a very thin atmosphere and little or no
current volcanism.
• There is no evidence of plate tectonics on either Venus or
Mars. On Venus, there is vigorous convection in the planet’s
interior, but the crust is too thin to move around in plates;
instead, it wrinkles and flakes. On Mars, the planet’s smaller
size means the crust has cooled and become too thick to
undergo subduction.
Key Ideas
• Volcanoes on both Venus and Mars were produced by hot
spots in the planet’s interior.
• The entire Venusian surface is about 500 million years old and
has relatively few craters. By contrast, most of the Martian
surface is cratered and is probably billions of years old. The
southern highlands on Mars are the most heavily cratered and
hence the oldest part of the planet’s surface.
Key Ideas
• The Atmospheres of Venus and Mars: Both planetary
atmospheres are over 95% carbon dioxide, with a small
percentage of nitrogen.
• The pressure at the surface of Venus is about 90 atmospheres.
The greenhouse effect is very strong, which raises the surface
temperature to 460°C. The pressure at the surface of Mars is
only 0.006 atmosphere, and the greenhouse effect is very
• The permanent high-altitude clouds on Venus are made
primarily of sulfuric acid. By contrast, the few clouds in the
Martian atmosphere are composed of water ice and carbon
dioxide ice.
Key Ideas
• The circulation of the Venusian atmosphere is dominated by
two huge convection currents in the cloud layers, one in the
northern hemisphere and one in the southern hemisphere.
The upper cloud layers of the Venusian atmosphere move
rapidly around the planet in a retrograde direction, with a
period of only about 4 Earth days.
• Weather on Mars is dominated by the north and south flow of
carbon dioxide from pole to pole with the changing seasons.
This can trigger planetwide dust storms.
Key Ideas
• Evolution of Atmospheres: Earth, Venus, and Mars all began
with relatively thick atmospheres of carbon dioxide, water
vapor, and sulfur dioxide.
• On Earth, most of the carbon dioxide went into carbonate
rocks and most of the water into the oceans. Ongoing plate
tectonics recycles atmospheric gases through the crust.
• On Venus, more intense sunlight and the absence of plate
tectonics led to a thick carbon dioxide atmosphere and a
runaway greenhouse effect.
• On Mars, a runaway icehouse effect resulted from weaker
sunlight and the absence of plate tectonics.
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